In warm periods, air-cooling systems work to bring building temperatures to a comfortable level. These cooling systems are relatively inefficient and consume large amounts of energy to maintain the proper indoor temperature during warm periods. The amount of heat in the building that must be overcome is known as the heat load. The warmer it is outside and the less insulated a building is, the greater a heat load will typically be. The greater heat load results in more energy required to bring the indoor temperature of the building to a comfortable level.
Every window in a building adds to the heat load for several reasons. First, it has a low insulation value, so warm outdoor temperatures move fairly quickly indoors through windows, as opposed to walls. Second, windows often enable air to squeeze through small leaks, which also enable faster movement of warm air into buildings. Finally, another contribution to the heat load comes from the direct sun that shines through a window heats up the floor, walls, furniture, and other items in a building, which in turn heat the surrounding air. This is known as passive heating, or greenhouse effect, and is a substantial contributor to the cooling energy consumption during warm periods.